The present invention relates to a method and system of loop transmission utilizing specific patterns.
Generally, in this type of loop transmission system, only one specific pattern circulates through a loop, and a fault in the specific pattern is so serious that operation of the entire loop system is affected by the faulty specific pattern.
In a prior art loop transmission system utilizing the specific pattern, the master station sends only a specific pattern which is utilized for exchanging in each slave station, and abnormality of the specific pattern is monitored by simply monitoring the time for the specific pattern to circulate through one loop. This monitoring fails to detect faulty disappearance of the specific pattern because in such an event, time-up of the monitoring timer results. In addition, if a plurality of specific patterns are generated by fault of the specific pattern sending device of the master station and they are circulated through the loop, these faulty specific patterns cannot be detected. In particular, when a GO AHEAD pattern (hereinafter simply referred to as GA) is used as the specific pattern, disappearance of the GA can be detected by monitoring the maximum delay time for frame loop circulation T.sub..beta. of the GA since the maximum time T.sub..beta. is a known value that can be calculated from the data transmission rate, the bit delay time in each station and the system construction. Namely, the monitoring station restarts the maximum time T.sub..beta. each time the GA is detected and judges that disappearance of the GA occurs when the maximum time T.sub..beta. is up. But occurrence of doubling and disappearance of the GA within the maximum time T.sub..beta. cannot be detected.
The number of frequencies of occurrence of a doubled GA during circulation of the normal GA through one loop is unknown. Also, it is impossible to discriminate whether the GA detected by the monitoring station is the normal GA or the doubled GA. In the event of occurrence of the doubled GA, the interval between one GA detection and the other GA detection by the monitoring station is decreased and the monitoring station simply judges that the decreased interval is due to the absence of data sent from the slave station, thus failing to detect the doubled GA. Briefly, the prior art specific pattern monitoring system is disadvantageous in that it can detect disappearance of the specific pattern but cannot detect doubling thereof. Today, the number of stations being coupled to the loop is increasing and the detection of a doubled GA is of great significance.
On the other hand, each station erases, at the unit of 8 bits, the frame sent by its own station which has been exchanged at a communicating station and thereafter returned to the originating station. Each station also performs an operation for inserting "0" and erasing "0" in order to maintain the transparence of data as will be described later. Under these circumstances, if "0" is inserted in the sending frame, the sending frame length will be elongated by bits corresponding to the number of "0" data inserted. As a result, the sending frame is divided by 8, leaving behind a surplus which turns into an unerased part of a frame that causes various kinds of abnormality in the system.
The originating station monitors the originating station address in the received frame to judge whether or not the received frame is the frame it has sent, and erases the received frame which is identical with the frame it has sent. If the originating station address undergoes bit errors and turns into a non-existent station address, the bit error address will turn into a destination lost frame which cannot be erased by any existing station and which permanently circulates through the loop. The destination lost frame would cause another bit error which would be received by an uncertain station, thus raising an abnormality in the system.